High-mol.-wt levan injected locally inhibits the growth of Lewis lung carcinoma in C57BL mice. The inhibition is dependent on the number of tumour cells injected and on the dose of levan. The inhibition decreases tumour incidence and size as well as prolonging survival. The polysaccharide is most effective when injected daily beginning on the day of tumour-cell inoculation. Treatment begun on later dates is less effective. Treatment begun one day before tumour-cell inoculation enhances tumour growth. Histological studies showed that levan induces an intense polymorphonuclear (PMN) reaction followed by accumulation of vacuolated, levan-laden macrophages. Both PMN and activated macrophages seemed to have an inhibitory effect upon the growth of the tumour. The effector role of PMN was not explained by the histological study. Tumour cells in close contact with levan-laden macrophages appeared mostly necrotic. Administration of levan begun one day before tumour-cell inoculation produced a similar reaction, but the infiltrating cells did not appear to approach and damage the tumour cells.